1. Field
This disclosure relates generally to reduction of oscillations occurring during extreme ultraviolet light generation and more particularly to droplet generation.
2. Description of Related Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to generate EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be generated by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site within an LPP EUV source plasma chamber.
When operating an EUV light source in a firing pattern having bursts lasting 1 ms or longer than super-pulse 1 ms, the resulting EUV energy is subject to sudden-onset oscillations. These oscillations occur at deterministic frequencies that are a function of the droplet time-of-flight and other parameters of the LPP EUV system (e.g., pressure and surrounding gas flow velocity) from a nozzle to an irradiation site in the plasma chamber. Current LPP EUV systems actively compensate for the oscillations by actuating source laser amplitude or by actuating timing of a source laser.